1. Technical Field
The present invention relates to an apparatus for speed control of a DC motor.
More particularly, the present invention concerns a apparatus for speed control of a DC motor capable of operating the DC motor with a larger values of maximum controlling torque and starting torque than the prior arts.
2. Background
Conventional speed control apparatus for a small DC motor, so-called electronics governor apparatus, generally is designed to operate in a manner that a counter-electromotive force which is in proportion to the speed, of the motor is taken out across both ends of motor winding and is compared with a reference voltage thereby to produce a difference of the two voltage, then the DC motor is controlled by utilizing the difference of the two voltage. In such conventional apparatus, the problem is that the speed control is made by the difference voltage, which is between the reference voltage and the voltage obtained across both ends of the motor winding. In general, the latter voltage obtained across the motor winding is not exactly the counter electromotive force per se, but a voltage also including a voltage drop mainly due to ohmic resistance of the motor winding. Therefore, it has been general to construct the circuit to include a resistor bridge circuit in order to provide an accurate speed controlling by taking out and utilizing the pure counter-electromotive force across both ends of the motor winding. In the speed control apparatus including such resistor bridge circuit, it is an important problem that the resistance values of the resistors should be accurate as designed. Recently, many electronics circuit has been semiconductorized or formed on an integrated circuit, and such speed control apparatuses are also made in the form of semiconductor integrated circuit. However, since making of the accurate resistance values of the bridge resistors by semiconductor diffused regions is generally difficult, these bridge resistors are usually provided as an external resistors which is connected to the integrated circuit. Such use of external resistors with the integrated circuit inevitably requires a number of external connection wires from the integrated circuit.
In order to remove the abovementioned inconvenience of a large number of wire connections, a conventional example of an integrated circuit type speed control apparatus shown in FIG. 1 has been already proposed and known.
In the known speed control apparatus of FIG. 1, the parts encircled by the dotted lines are formed on a monolithic integrated circuit, wherein 1 designates a controlling circuit, 2 a reference voltage generator, 3 a current limiting circuit, 4 a constant current circuit and 5 a current dviding circuit. A power source terminal 6, an output terminal 7 and an adjusting resistor terminal 8 for connecting an adjusting resisor 10 are provided as external connection terminals of the IC. Across the power source terminal 6 and the ground is connected a voltage source (not shown), across the output terminal 7 and the ground is connected a motor 9, and across the adjusting resistor terminal 8 and the ground is connected the adjusting resistor 10, by changing the resistance of which a desired speed of the motor is set.
In the abovementioned conventional speed control apparatus, the circuit operation is made in a manner that a reference voltage Vref which is proportional to the counter-electromotive force Ea of the DC motor is generated across the output terminal 7 and the adjusting resisjtor terminal 8, and a voltage drop in the adjusting resistor 10 is changed so as to be equal to the reference voltage Vref. For instance, when a voltage drop by an internal resistance of the DC motor 9 is changed by an external effect, the operation is made in a manner that by changing a current flowing through the adjusting resistor 10 by means of the current dividing circuit 5, the reference voltage Vref is hold to be equal to the counter-electromotive force Ea, so that the fluctuations of the revolving speed of the DC motor is suppressed.
When speed control of a DC motor is made by the above-mentioned speed control apparatus, there is such problems that it is difficult to obtain a large starting torque, and therefore, it takes a considerable time until the revolution speed of the motor reaches a predetermined value, and the the maximum controlling torque is small.
The present invention is made in view of the abovementioned problems of the conventional speed control appratus.